Design of a wide-stopband bandpass filter using stepped impedance resonators

Abstract

In this paper, a compact bandpass filter (BPF) with improved stopband characteristics using stepped impedance resonators (SIRs) is proposed. The filter consists of two quarter-wavelength SIRs with radial stubs and a PI-network of capacitance, which is used to suppress the upper frequency spurious signals. An odd- and even-mode analysis is adopted considering the structure’s symmetry. A new BPF centered at 1.57 GHz has been designed and fabricated to verify the validity of the proposed method. Measured results are in good agreement with the electromagnetic simulation. The frequency selectivity was enhanced via introducing two finite transmission zeros (TZs) near the passband, located at 1.21 GHz with 43.5 dB rejection and 3.10 GHz with 62.3 dB rejection, respectively. The spurious frequencies of the upper stopband are suppressed better than 20 dB from 2.07 to 16.25 GHz. And the circuit only occupies 0.09 λ_g × 0.09 λ_g.

Keywords:

• bandpass filter
• wide-stopband
• stepped impedance resonator
• transmission zeros

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51172034) and the Fundamental Research Funds for the Central University of UESTC (ZYGX2010J120).